Citric acid is used as a food acidulant, and in pharmaceutical, industrial and detergent formulations. The increased popularity of liquid detergents formulated with citric acid has been primarily responsible for growth of worldwide production of citric acid to about 700 million pounds per year which is expected to continue in the future. Furthermore, in many applications, such as in food, beverages, pharmaceuticals and liquid detergents, the salts of citric acid, e.g., sodium-, potassium- and ammonium-citrate are used.
Citric acid is produced by a submerged culture fermentation process which employs molasses as feed and the microorganism, Aspergillus Niger. The fermentation product will contain carbohydrates, amino acids, proteins and salts as well as citric acid, which must be separated from the fermentation broth.
There are two technologies currently employed for the separation of citric acid from fermentation broths containing the same. The first involves calcium salt precipitation of citric acid. The resulting calcium citrate is acidified with sulfuric acid. In the second process, citric acid is extracted from the fermentation broth with a mixture of trilauryl-amine, n-octanol and a C.sub.10 or C.sub.11 isoparaffin. Citric acid is reextracted from the solvent phase into water with the addition of heat. Both techniques, however, are complex, expensive and they generate a substantial amount of waste for disposal.
The patent literature has suggested a possible third method for separating citric acid from the fermentation broth, which involves membrane filtration to remove raw materials or high molecular weight impurities and then adsorption of contaminants onto a nonionic resin based on polystyrene or polyacrylic resins and collection of the citric acid in the rejected phase or raffinate and crystallization of the citric acid after concentrating the solution, or by precipitating the citric acid as the calcium salts then acidifying with H.sub.2 SO.sub.4, separating the CaSO.sub.4 and contacting cation-and anion-exchangers. This method, disclosed in European Published Application No. 151,470, Aug. 14, 1985, is also a rather complex and lengthy method for separating the citric acid.
U.S. Pat. No. 4,323,702, corresponding to British Patent No. 2,064,526A discloses that citric acid may be adsorbed from aqueous solutions thereof onto a porous, crosslinked pyridine group-containing polymer made from an ethylenically unsaturated monomer, e.g., a vinyl pyridine monomer. However, the polymer is used as a free base and the acid is desorbed by an organic solvent, such as an alcohol or a ketone. Furthermore, there is not teaching or suggestion to separate citric acid from a fermentation broth containing other organic materials.
The establishment of pH below the pk value in an adsorbent separation of citric acid from other acids is disclosed in U.S. Pat. No. 2,664,441, but there is no suggestion of or rationale for the application thereof to the instant separation of citric acid from mixtures thereof with non-acidic components.
In Kulprathipanja Patent 4,720,579, referred to above, citric acid is separated from a fermentation broth by using an adsorbent comprising a neutral, non-iogenic, macroreticular, water-insoluble, crosslinked styrene-poly(vinyl)benzene and a desorbent comprising water and, optionally, acetone with the water. The pH of the feed is adjusted and maintained below the first ionization constant (pKa.sub.1) of citric acid to maintain selectivity. However, separation of citric acid by the process disclosed therein has the drawback that it is an extractive process and requires a substantial volume of desorbent liquid and amount of time to recover the citric acid from the adsorbent. One aspect of that invention was in the discovery that complete separation of citric acid from salts and carbohydrates with neutral resins is only achieved by adjusting and maintaining the pH of the feed solution lower than the first ionizaton constant (pKa.sub.1) of citric acid (3.13).
The degree to which the pH must be lowered to maintain adequate selectivity appears to be interdependent on the concentration of citric acid in the feed mixture; i.e., the pH is invesely dependent on the concentration. As concentrations are decreased below 13% to very low concentrations, the pH may be near the pKa.sub.1 of citric acid of 3.13; at 13%, the pH may range from 0.9 to 1.7; however, at 40% citric acid feed concentration, the pH mut be lowered to at least about 1.2 or lower. At higher concentrations, the pH must be even lower; for example, at 50% citric acid, the pH must be at or below 1.0. Another aspect of that invention was the discovery that the temperature of separation can be reduced by the addition of cetone, or other low molecular weight ketone, to the adsorbent; the higher temperatures associated with adsorbent breakdown can thus be avoided.
The invention disclosed in my parent application Ser. No. 121,830, filed Nov. 16, 1987, also relates to a process for separating citric acid from a feed mixture comprising a fermentation broth employing water-insoluble, macroreticular or gel, weakly basic anionic exchange resin possessing tertiary amine functional groups or pyridine functional groups having a crosslinked acrylic or styrene resin matrix.
Accordingly, it is an object of the present invention to separate citric acid from the impurities in a fermentation broth in a simple and economic process by adsorbing citric acid on an adsorbent and recovering the citric acid in the form of its salts. Succinctly stated, the citric acid is adsorbed selectively by the adsorbent, and the impurities are removed by elution with a weak desorbent, such as water, the adsorbed citric acid is converted to its salt by reaction with an aqueous alkaline solution, and the salt is spontaneously eluted by the aqueous solution.